This invention relates to a process for the upgrading of hydrocarbon streams. It more particularly refers to a process for upgrading gasoline boiling range petroleum fractions containing substantial proportions of sulfur impurities. The process involves integration of a first stage hydrotreating of a sulfur-containing cracked petroleum fraction in the gasoline boiling range and a second stage conversion of the hydrotreated intermediate product over a catalyst comprising a molecular sieve.
Catalytically cracked gasoline forms a major part of the gasoline product pool in the United States. It is conventional to recover the product of catalytic cracking and to fractionate the product into various fractions such as light gases; naphtha, including light and heavy gasoline; distillate fractions, such as heating oil and diesel fuel; lube oil base fractions; and heavier fractions. A secondary source of cracked gasoline is from thermal processes, such as coking or visbreaking.
A large proportion of the sulfur in gasoline is present in the catalytically cracked gasoline component. Such sulfur results from the sulfur content of the petroleum fractions being catalytically cracked. The sulfur impurities may require removal, usually by hydrotreating, in order to comply with product specifications or to ensure compliance with environmental regulations both of which are expected to become more stringent in the future, possibly permitting no more than about 30-50 pp sulfur in motor fuel gasolines, based on the weight of the gasoline. Low sulfur levels can contribute to reduced emissions of CO, NOx and hydrocarbons.
In FCC or TCC gasoline hydrotreating, the gasoline is contacted with a suitable hydrotreating catalyst at elevated temperature and somewhat elevated pressure in the presence of a hydrogen atmosphere. One suitable family of catalysts which has been widely used for this service is a combination of a Group VIII and a Group VI element, such as cobalt and molybdenum, on a suitable substrate, such as alumina. After completion of hydrotreating, the product may be fractionated, or flashed, to release the hydrogen sulfide and light hydrocarbons (e.g., those having a molecular weight below about C5, xe2x80x9cC5xe2x88x92xe2x80x9d) and to collect the sweetened gasoline.
Cracked naphtha, as it comes from a catalytic or thermal conversion process and without any further treatments, such as purifying operations, has a relatively high octane number, due, in part, to the presence of olefinic components. As such, cracked gasoline is an excellent contributor to the gasoline pool, providing a large quantity of product at a high blending octane number. In some cases, this fraction may contribute as much as up to half the gasoline in the refinery pool. In special situations, where a refinery has no catalytic reformer, the cracked naphtha may represent as much as 80% of the refinery""s gasoline.
Hydrotreating of any of the sulfur-containing fractions of cracked gasoline may lead to a reduction in the olefin content. However, octane number loss may be diminished by hydrotreating only the heaviest, most sulfur-rich and olefin-poor portion of the FCC gasoline. But if the future pool sulfur specification were reduced, an increasing amount of lighter boiling, olefin-rich, gasoline would be hydroprocessed, and the resulting octane number penalty would increase dramatically due to olefin saturation in these lighter gasoline fractions. The decrease in octane number which takes place as a consequence of sulfur removal by hydrotreating creates a tension between the need to produce gasoline fuels with sufficiently high octane number and the need to produce lower sulfur fuels.
Methods have been proposed for offsetting pool octane number reductions which might occur if severely hydrotreated, wide-cut FCC gasoline was introduced into the pool. Catalytic reforming increases the octane of virgin and hydrocracked naphthas by converting at least a portion of the paraffins and cycloparaffins to aromatics in these very low olefin content feeds. Reforming severity might be boosted to further increase the octane of the reformate going into the gasoline pool, thereby offsetting the negative impact on the pool from blending hydrotreated wide cut FCC gasoline. This approach, however, has two limitations. First, reformate yield declines as severity is increased which could negatively impact the total gasoline pool volume. Second, as already noted, aromatization reactions account, to a large degree, for the octane enhancement in reforming. However, specifications limit the amount of aromatics, particularly benzene, that may be present in the gasoline. It has therefore become desirable, as far as is feasible, to create a gasoline pool in which a greater portion of the octane number is contributed by non-aromatic components.
Instead of increasing reforming severity, post-reforming processes for increasing octane have been proposed such as those described in U.S. Pat. Nos. 3,767,568 and 3,729,409 in which the octane is increased by treatment of the reformate with ZSM-5. These processes, however, also can reduce reformats yield as severity is increased and thus impact overall gasoline pool volume. Instead of trying to use reforming or post-reforming approaches to compensate for pool octane losses potentially arising from the introduction of large amounts of hydrotreated FCC gasoline, it may be preferable to pursue strategies involving processing of the FCC gasoline itself. These can seek either to minimize octane loss during hydroprocessing or to achieve octane recovery in the hydroprocessed product.
Proposals have been made for removing sulfur impurities while retaining the high octane contributed by the olefins. For example, U.S. Pat. No. 3,957,625 discloses a method of removing sulfur from only the heavy fraction of a catalytically cracked gasoline by hydrodesulfurization, since the sulfur impurities tend to concentrate in the heavy fraction, while retaining the octane contribution from the olefins which are found mainly in the lighter fraction. Other methods have been proposed, which rely upon catalyst selection for selective hydrodesulfurization relative to olefin saturation, for example, by the use of a magnesium oxide support instead of the more conventional alumina. U.S. Pat. No. 4,049,542, for instance, discloses a process in which a copper catalyst is used to desulfurize an olefinic hydrocarbon feed such as catalytically cracked light naphtha.
Other processes for enhancing the octane rating of catalytically cracked gasolines have also been proposed in the past. For example, U.S. Pat. No. 3,759,821 discloses a process for upgrading catalytically cracked gasoline by fractionating it into a heavier and a lighter fraction and treating the heavier fraction over a ZSM-5 catalyst, after which the treated fraction is blended back into the lighter fraction. Another process in which the cracked gasoline is fractionated prior to treatment is described in U.S. Pat. No. 4,062,762 which discloses a process for desulfurizing naphtha by fractionating the naphtha into three fractions each of which is desulfurized by a different procedure, after which the fractions are recombined.
In cases where the gasoline pool sulfur specifications will not be met by processing only the heaviest, sulfur-rich olefin-poor portion of the FCC gasoline, the lighter components may also require treating to achieve acceptable sulfur levels. However, the octane number loss associated with hydroprocessing, or yield loss associated with processes aimed at recovering octane number losses, can increase dramatically with a widening of the boiling point range of the gasoline feed being treated.
Consequently, it would be desirable to develop cost-effective methods for preserving gasoline yield and octane number while removing sulfur from the relatively olefin-rich light and mid-range portions of the FCC gasoline pool.
Accordingly, an improved process has been developed for catalytically desulfurizing cracked fractions in the gasoline boiling range for reducing sulfur levels without substantially reducing the octane number. It has been discovered that a catalyst which includes at least one of a class of molecular sieve synthetic materials belonging to the MCM-22 family containing a metal component is beneficial in the gasoline upgrading process.
In one embodiment, a sulfur-containing cracked petroleum fraction in the gasoline boiling range is hydrotreated, in a first step, under conditions which remove at least a substantial proportion of the sulfur. The hydrotreated intermediate product is then treated, in a second step, by contact with a catalyst system of acidic functionality which comprises at least one of a class of molecular sieve materials belonging to the MCM-22 family and a metal component, preferably selected from the transition elements of the 4th or 5th period of the Periodic Table, under conditions which convert and substantially saturate the olefins contained in, and formed during processing of, the hydrotreated gasoline intermediate product fraction to provide a gasoline product fraction which has a higher octane value than the octane number of the gasoline fraction of the intermediate product.
Accordingly, an olefin-rich, sulfur-containing, cracked hydrocarbon stream as may be obtained, for example, from a catalytic or thermal cracking process, may be upgraded by contacting the stream with a catalytically effective amount of a hydrodesulfurization catalyst in a first reaction zone, operating under a combination of elevated temperature, elevated pressure and an atmosphere containing hydrogen, under catalytic conversion conditions, to produce an intermediate product containing a liquid fraction which has a reduced sulfur content and a reduced octane number as compared to the cracked hydrocarbon stream, and thereafter contacting at least the gasoline boiling range portion of the intermediate product in a second reaction zone with a catalytically effective amount of a second catalyst system having acidic functionality containing at least one molecular sieve belonging to the MCM-22 family and a metal component, preferably selected from the transition elements of the 4th or 5th period of the Periodic Table, under conditions which convert and substantially saturate the olefins contained in, and formed during processing of, the gasoline fraction of the intermediate product to provide a product with a gasoline fraction having a higher octane number than the octane number of the gasoline fraction of the intermediate product.
In one embodiment, the process may be utilized to desulfurize light and full range naphtha fractions while enhancing at least one of yield and octane number compared to processes employing other catalysts to restore octane number lost during hydrotreating.